No, it’s not.
Spanish at 15
I started learning Spanish at 15 and I would say I’m fluent. Could definitely keep improving at it, but I understand about 95% of what I hear and more sometimes (as well as less other times). Can express myself pretty well.
German at 19
I started learning German when I was 19 and can have conversations with people, my level is a bit lower than compared to my Spanish. At one point my German was better than my Spanish.
Chinese at 28
I started learning Chinese when I was 28. Not fluent yet, but I could be with more work. Have no reason to doubt I could get there.
Would I be better at any of these languages if I started earlier? Probably.
At the same time, I became better at German compared to Spanish and I started learning it later. When you start something is not the only factor in determining your ultimate ability.
“Child prodigies” with music sometimes stop playing music.
For me, I did not consistently put in work learning Spanish and German. Could maybe even make a graph of that.
I could speak either Spanish or German much better right now if I had approached it differently. But I can speak both in a way that could be described as fluent.
I would recommend starting now and dedicating yourself to anything you want to learn.
But don’t doubt you can do it.